The present invention relates to exhaust gas sensors, and more particularly to exhaust gas sensors used for lean air/fuel ratio applications.
Exhaust gas sensors are well known for use in automotive and other industries. Certain exhaust gas sensors, such as sensors used in high performance race cars, ovens, boilers, and diesel vehicles, must be capable of performing well in lean air/fuel ratio applications where the normalized air/fuel ratio, λ, is greater than 1. λ is determined based on the following equation:
      λ    =                            (                      A            /            F                    )                actual                              (                      A            /            F                    )                ideal                        where      ⁢              :            ⁢              A        /        F              =                  Air        /        fuel            ⁢                          ⁢      ratio      
These lean-application exhaust gas sensors typically have a low air-voltage, which is the voltage measured between the signal wire lead and the ground lead when the sensor is operating in an air-only atmosphere (i.e., no fuel is present). The air-voltage can be considered an indicator of the sensor's sensitivity. Three known and commonly requested air-voltage ranges for lean-applications sensors are about −8.5 mV to about −12 mV, about −12 mV to about −15 mV, and about −15 mV to about −18 mV.
FIG. 1 illustrates a prior art lean-application exhaust gas sensor 10. The illustrated prior art sensor is known as a 3-wire sensor because it includes one signal wire 14 and two heater wires 18 (only one shown in FIG. 1) housed in the sensor 10. There is no ground wire incorporated in the sensor 10. As shown in FIG. 2, the sensor 10 is connected to a wire harness having two heater leads 22 electrically coupled to the respective heater wires 18, one signal lead 26 electrically coupled to the signal wire 14, and a ground lead 30 that is welded to a metal cap 34, thereby grounding the ground lead 30 via the electrical connection with the metallic housing 38, the sleeve 42, and the seal ring 46 (see FIG. 1) of the sensor 10. The ground electrode of the ceramic sensor element 54 is also connected to the housing 38 as the ground. The air-voltage of the sensor 10 is measured by measuring the voltage across the signal lead 26 and the ground lead 30.
FIG. 3 illustrates the circuit model for the sensor 10. Reference numeral 50 represents the signal supplied by the signal wire 14, reference numeral 54 represents the ceramic sensor element, and reference numeral 58 represents the signal supplied by the ground lead 30. The air-voltage is the voltage measured between the positive and negative terminals shown in the circuit model when the sensor 10 is in air with no fuel present.